Anesthesia with xenon as an anesthetic gas has been described in the specialist medical literature for many years now. There are a number of medical advantages compared with laughing gas (N.sub.2 O) which is customary nowadays. However, the widespread introduction of xenon for this application has hitherto been impeded by the very much higher materials costs.
Developments in recent years have drastically reduced this difference in costs. These include improved anesthetic methods with low gas consumption (low-flow technique; minimal-flow technique) and methods for recovering the exhaled xenon mixture which make it possible to recycle the active component xenon in the anesthetic gas circulation (DE 44 11 533 C1).
Hitherto, admixture of anesthetic gas components has taken place manually.
DE 37 12 598 A1 describes an inhalation anesthetic machine. Besides other anesthetic gases xenon is mentioned as anesthetic gas. The machine has a gas analyzer, which is not characterized in detail.
DE 36 35 004 A1 describes a mass spectrometer for monitoring respiratory gases, the mass spectrometer measuring the carbon dioxide level.
Analytical determination of the anesthetic gas xenon is difficult as it is an inert gas. Gas analyzers customary in anesthetic machines are unsuitable for quantitative determination of xenon.
When xenon is used as anesthetic gas it is indispensable to recycle xenon from the exhaled gas for cost reasons. When xenon is recycled into the ventilation gas (inspiration side), satisfactory and reliable measurements of the gas mixture composition in the inspiration branch is indispensable. On the one hand, the gas mixture composition delivered from the recovery must be monitored continuously so that it is possible permanently to ensure the gas quality on recycling into the breathing circulation, and to switch over immediately to an auxiliary supply (for example gas cylinder) in the event of faults in the machine. On the other hand, the composition of the anesthetic gas in the breathing circulation must be continuously followed so that the clinician can monitor and control the progress of anesthesia individually. Besides the active component xenon and the respiratory component oxygen, it is additionally necessary to monitor the nitrogen content which is included as medically acceptable residual impurity from the recovery and whose accumulation in the breathing circulation must be limited. Besides reliable monitoring of the gas composition of inspired gas (gas for inhalation) and expired gas (gas exhaled by the patient), it is an object of the invention to automate the mixing of the respiratory gas components and the admixing of recycled anesthetic gas.
The invention now relates to an anesthesia system with mass spectrometer for quantitative measurement of at least one gas component in the ventilation gas, exhaled gas or recycled anesthetic gas-containing gas.
Mass spectrometers can in general be connected via a membrane or capillary to a gas stream to be analyzed. Coupling via a membrane has the disadvantage of a large gas consumption (for example around 5 1/h). Coupling capillaries is advantageous. The loss of gas can be reduced to about 0.5 1/h in this way. The capillary can consist of plastic, metal or glass. Metal capillaries are preferred, especially with prolonged measurement periods and lengthy capillaries. The capillaries may be employed, for example, with a length of from 6 to 10 meters. This permits flexibility in the site for setting up the mass spectrometer.
The mass spectrometer in the anesthesia system according to the invention is preferably used for simultaneous quantitative measurement of the gas components oxygen, anesthetic gas (for example xenon) and nitrogen in the inspired gas, expired gas or recycled anesthetic gas-containing gas. The measurement can be extended to other gas components such as carbon dioxide.
The anesthesia system is advantageously designed so that the mass spectrometer is connected via control valves to the gas lines for inspired gas, expired gas and, where appropriate, recovered anesthetic gas or recycled respiratory gas.
The anesthesia system contains at least one mass spectrometer which can be integrated into the anesthetic machine, or can be set up in the direct vicinity of the anesthetic machine (for example as so-called backpack) or some meters away from the anesthetic machine (for example in an adjacent room). The mass spectrometer is functionally connected to the anesthetic machine. The mass spectrometer can both monitor the anesthetic gas circulation and measure the gas fed in from the xenon recovery via two measurement channels simultaneously or alternately in short cycles.
A suitable mass spectrometer is a commercial apparatus supplied by Leybold AG (Cologne) with the designation Ecotec 500, which has a very compact design and already has a computer interface for transmitting the measured signal. This commercial apparatus can be employed without the elaborate apparatus peripherals hitherto customary with mass spectrometers and, when the sampling points are appropriately arranged, provides real-time measured data. The mass spectrometer is designed for the mass range from 1 to 100 atomic mass units. The restriction to this mass range makes a very compact design possible. Xenon has an atomic mass of 132 and cannot be determined directly with such an apparatus. The problem has been solved by doubly ionizing xenon (formation of Xe.sup.2+) for the measurement.
The mass spectrometric measurements usually take place with clock-pulse rates of 1 measurement/second. The clock-pulse rate can also be chosen to be shorter or longer.
The monitoring of the recovered anesthetic gas-containing gas on entry into the anesthetic machine means that the recovery system is indirectly monitored from the anesthetic machine. Despite this indirect monitoring, there are no restrictions of any kind on the possibility of reacting to faults in the recovery operation, because the recovered gas is monitored exactly where it is used.
The computer normally present in an anesthetic machine furthermore makes it possible to use the analytical data for process control. Recovered anesthetic gas (for example xenon) and fresh anesthetic gas (from the gas cylinder, anesthetic gas source) can be automatically mixed via a valve control or flow regulator so that the anesthesia parameters preselected by the clinician are set up. Since the losses of xenon which regularly occur in the system must be compensated by adding fresh xenon, automatic control of the gas flows on the basis of the analytical results offers great advantages. In addition, the computer-assisted control of the mass spectrometer makes it possible continuously to document the gas-related anesthesia parameters and thus meets the requirement for continuous documentation of the progress of anesthesia.
It is furthermore possible to monitor and document, through a sampling point in the expiratory branch of the anesthesia circuit, which mixture of substances is fed from the anesthetic machine into the recovery.
If several anesthetic machines are coupled to a single recovery system, it is possible for each mass spectrometer independently of the others to interrupt the gas supply from the recovery in the event of faults or else to block the recovery system entirely. If, for reasons of simplicity, monitoring of the anesthesia circuit is dispensed with, the recovery can also be monitored by a single mass spectrometer which, depending on the number of connected anesthetic machines, is positioned between the recovery gas outlet and the point of branching to the anesthetic machines.
The described analytical configuration can also be used for anesthesia with conventional laughing gas. It is therefore possible to monitor the anesthesia circuit irrespective of the anesthetic gas chosen. This is particularly advantageous when modern anesthetic machines permit operation with xenon or laughing gas as selected. As a rule, no recovery unit is employed for anesthesia with laughing gas. If, for environmental protection or worker safety reasons, in future it becomes necessary for the outflowing laughing gas mixture to be destroyed on site, it will also be possible for such a disposal unit likewise to be monitored, controlled or documented with a mass spectrometer.
The choice of a mass spectrometer with reduced mass range and the described linkage into the system of anesthetic machine and recovery system entirely meet the functional requirements described above and, furthermore, can be implemented at costs which are distinctly below those to be expected with conventional mass spectrometers. This means that the described anesthesia system is reasonably priced and, furthermore, permits economic operation of xenon anesthesia.